Quantum phase transition in a single-molecule quantum dot

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Abstract

Quantum criticality is the intriguing possibility offered by the laws of quantum mechanics when the wave function of a many-particle physical system is forced to evolve continuously between two distinct, competing ground states1. This phenomenon, often related to a zero-temperature magnetic phase transition, is believed to govern many of the fascinating properties of strongly correlated systems such as heavy-fermion compounds or high-temperature superconductors1. In contrast to bulk materials with very complex electronic structures, artificial nanoscale devices could offer a new and simpler means of understanding quantum phase transitions2,3. Here we demonstrate this possibility in a single-molecule quantum dot, where a gate voltage induces a crossing of two different types of electron spin state (singlet and triplet) at zero magnetic field. The quantum dot is operated in the Kondo regime, where the electron spin on the quantum dot is partially screened by metallic electrodes. This strong electronic coupling between the quantum dot and the metallic contacts provides the strong electron correlations necessary to observe quantum critical behaviour. The quantum magnetic phase transition between two different Kondo regimes is achieved by tuning gate voltages and is fundamentally different from previously observed Kondo transitions in semiconductor and nanotube quantum dots4,5. Our work may offer new directions in terms of control and tunability for molecular spintronics6.

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Figure 1: Quantum phase transition, device and conductance characteristics.
Figure 2: Magnetic field and gate-induced singlet–triplet transition.
Figure 3: Singlet–triplet quantum phase transition and universal scaling.
Figure 4: Kondo effects in the singlet and triplet states.

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Acknowledgements

We acknowledge E. Eyraud and D. Lepoittevin for discussions and technical contributions regarding electronics and dilutions. We thank E. Bonet, T. Crozes and T. Fournier for lithography development, and C. Winkelmann, T. Costi and L. Calvet for discussions. The sample used in the investigations was made in the NANOFAB facility of the Néel Institut. This work is partially financed by ANR-PNANO, Contract MolSpintronics.

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Correspondence to Franck Balestro.

Supplementary information

Supplementray Information

The file contains Supplementray Notes, Supplementray Fugures S1-S6 with Legends and additional references. The Supplementray Information includes the following parts: 1. Experimental setup ; 2. Fully-screened spin S=1/2 Kondo effect in a C60 quantum dot ; 3. Non-equilibrium singlet-triplet Kondo effect on the singlet side ; 4. Singlet-triplet transition: low versus very low temperature ; 5. Temperature dependence of the zero-bias conductance ; 6. Statistics and reproducibility of the results. (PDF 6050 kb)

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Roch, N., Florens, S., Bouchiat, V. et al. Quantum phase transition in a single-molecule quantum dot. Nature 453, 633–637 (2008) doi:10.1038/nature06930

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